I think that in strength Yamamoto's would be the strongest, but then again we have no clue what Gin's will be. In terms of beating multiple opponents, I think Aizen's would be stronger because his stats are exactly the same, and Kyoka Suigetsu already WTFPWNS. The vizard are like Russian Roulette so im not gonna say. Tuberculosis, and Mr. Perv are completely out of my guessing range BUT Tuberculosis warned Pervy not to use it in front of people. Now I am extremely partial to Urahara (Fapfapfap), and he told Renji his bankai wasn't for training so that puts on the mysterious effect. Unohana...I will always say this, I think her bankai will multiply her fan base by 20 to the 8th power. Kensei's I think isn't too good, as Wonderweiss came back from the fight un-scratched. But he's still hot.
So whattya you guys think about their bankai's?